Capping layer for improved deposition selectivity

ABSTRACT

The present disclosure, in some embodiments, relates to a method of forming an integrated chip. The method may be performed by forming a first capping layer on a dielectric structure over a substrate, and patterning the dielectric structure and the first capping layer to define cavities within the dielectric structure. A conductive material is formed within the cavities and a second capping layer is formed on the conductive material. An etch stop layer is formed along sidewalls and over an upper surface of the second capping layer. The etch stop layer has a first thickness over the first capping layer and a second thickness over the second capping layer. The first thickness is greater than the second thickness.

REFERENCE TO RELATED APPLICATIONS

This Application is a Continuation of U.S. application Ser. No. 15/151,616, filed on May 11, 2016, which is a Continuation of U.S. application Ser. No. 13/755,089, filed on Jan. 31, 2013 (now U.S. Pat. No. 9,396,990, issued on Jul. 19, 2016). The contents of the above-referenced Patent Applications are hereby incorporated by reference in their entirety.

BACKGROUND

The fabrication of integrated chips can be broadly separated into two main sections, front-end-of-the-line (FEOL) fabrication and back-end-of-the-line (BEOL) fabrication. FEOL fabrication includes the formation of devices (e.g., transistors, capacitors, resistors, etc.) within a semiconductor substrate. BEOL fabrication includes the formation of one or more metal interconnect layers comprised within one or more insulating dielectric layers disposed over the semiconductor substrate. The metal interconnect layers of the BEOL electrically connect individual devices of the FEOL to external pins of an integrated chip.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A-1B illustrate cross-sectional views showing formation of a back-end-of-the-line metal layer using a conventional metal capping process.

FIG. 2 is a flow diagram of some embodiments of a disclosed method of integrated chip processing that improves back-end-of-the-line reliability.

FIG. 3 a cross-sectional view of some embodiments of a back-end-of-the-line (BEOL) layer comprising a first capping layer configured to improve BEOL reliability.

FIG. 4 illustrates graphs showing exemplary cobalt selectivity achieved by a disclosed first capping layer comprising an extreme low-k (ELK) film.

FIG. 5 is a flow diagram of some embodiments of a method of integrated chip processing that improves back-end-of-the-line (BEOL) reliability.

FIGS. 6-13 illustrate cross-sectional views of some embodiments of an integrated chip (IC) whereon a disclosed method of integrated chip processing that improves back-end-of-the-line reliability is implemented.

DETAILED DESCRIPTION

The description herein is made with reference to the drawings, wherein like reference numerals are generally utilized to refer to like elements throughout, and wherein the various structures are not necessarily drawn to scale. In the following description, for purposes of explanation, numerous specific details are set forth in order to facilitate understanding. It will be appreciated that the details of the figures are not intended to limit the disclosure, but rather are non-limiting embodiments. For example, it may be evident, however, to one of ordinary skill in the art, that one or more aspects described herein may be practiced with a lesser degree of these specific details. In other instances, known structures and devices are shown in block diagram form to facilitate understanding.

FIGS. 1A-1B illustrates cross-sectional views, 100 and 108, showing formation of a back-end-of-the-line (BEOL) metal layer of an integrated chip (IC) using a conventional metal capping process.

As shown in cross-sectional view 100, a low-k dielectric layer 104 is formed over a semiconductor substrate 102. One or more metal layer structures 106 are formed within the low-k dielectric layer 104. As shown in cross-sectional view 108, after formation of the metal layer structures 106, a capping layer 110 (e.g., a cobalt capping layer) having a substantially uniform thickness is deposited over the low-k dielectric layer 104 and the metal layer structures 106. The capping layer 110 improves electromigration at the top interface of metal layer structures 106. An etch stop layer 112 may be subsequently deposited over the capping layer 110 prior to the formation of an overlying metal layer.

In modern technology nodes (e.g., 45 nm, 32 nm, 22 nm, 14 nm, etc.), low-k dielectric layers, having a low dielectric constant, are used to provide good electrical isolation between adjacent metal layer structures, allowing for spaces between adjacent metal layer structures to shrink and the density of an integrated chip to increase. However, low-k dielectric layers often have a high porosity. The high porosity of low-k dielectric layers leads to diffusion of atoms from the capping layer 110 into the underlying low-k dielectric layer 104. Such diffusion lead to reliability issues, such as poor time dependent dielectric breakdown (TDDB) and voltage breakdown (VBD).

Accordingly, the present disclosure relates to a method and apparatus for improving back-end-of-the-line (BEOL) reliability. In some embodiments, the method comprises forming an extreme low-k (ELK) dielectric layer comprising one or more metal layer structures over a semiconductor substrate. A first capping layer is deposited over the ELK dielectric layer at a position between the one or more metal layer structures, resulting in a planar surface having metal layer structures interspersed between the first capping layer. A second capping layer is then deposited over the one or more metal layer structures. The first capping layer has a high selectivity that limits interaction between the second capping layer and the ELK dielectric layer, reducing diffusion of the atoms from the second capping layer to the ELK dielectric layer and improving dielectric breakdown of the ELK dielectric layer.

FIG. 2 is a flow diagram of some embodiments of a method 200 of integrated chip processing that improves back-end-of-the-line (BEOL) reliability by reducing BEOL time dependent dielectric breakdown (TDDB) and voltage breakdown (VBD) caused by diffusion from a first capping layer into a dielectric material.

At act 202, a low-k dielectric layer is formed over a semiconductor substrate. In some embodiments, the low-k dielectric layer comprises an extreme low-k (ELK) dielectric layer with a high porosity having a dielectric constant in a range of between approximately 2.4 and 2.6 and a density in a range of between approximately 1.08 g/cm³ to approximately 1.15 g/cm³.

At act 204, a first capping layer is deposited over the low-k dielectric layer. In some embodiments the high density first capping layer has a density greater than that of the underlying low-k dielectric layer. The first capping layer may be deposited by a blanket deposition of a high density ELK film or a non-porous non-ELK film. In some embodiments, the high density ELK film comprises an ELK film (e.g., SiCO) having a dielectric constant in a range of between approximately 2.6 and 2.8. In other embodiments, the non-ELK film may comprise silicon nitride (SiN), silicon oxide (SiO), silicon carbon nitride (SiCN), or another similar material.

At act 206, a metal layer extending though the first capping layer is formed within the low-k dielectric layer. The resulting structure forms a substantially planar top surface of the substrate having exposed metal layer structures interspersed between the first capping layer.

At act 208, a second capping layer is selectively deposited. Surface properties of the first capping layer cause for the second capping layer to selectively accumulate to a greater thickness over the metal layer structures relative to the first capping layer, thereby providing for a high selectivity of between the first capping layer and the metal layer structures. For example, in some embodiments, the thickness of the second capping layer over the metal layer structures is greater than ten times the thickness of the second capping layer over the first capping layer. In some embodiments, the second capping layer may comprise cobalt.

Therefore, method 200 deposits a first capping layer that minimizes interactions between the second capping layer and the underlying low-k dielectric layer. By minimizing interactions between the second capping layer and the underlying low-k dielectric layer, diffusion from the second capping layer to the underlying porous dielectric material is mitigated, improving TDDB and VDB reliability.

FIG. 3 illustrates a cross-sectional view of some embodiments of a back-end-of-the-line (BEOL) layer 300 comprising a first capping layer configured to improve BEOL reliability.

As shown in BEOL layer 300, a first etch stop layer 302 is disposed over a semiconductor substrate 102. In some embodiments, the semiconductor substrate 102 may comprise one or more semiconductor devices. In various embodiments, the first etch stop layer 302 may comprise silicon nitride (SiN), silicon oxide (SiO), or silicon carbon nitride (SiCN), for example.

A low-k dielectric layer 304 is located over the first etch stop layer 302. In some embodiments, the low-k dielectric layer 304 comprises an extreme low-k (ELK) dielectric layer. The ELK dielectric layer may comprise a lower ELK dielectric layer 304 a and upper ELK dielectric layer 304 b. The lower ELK dielectric layer 304 a comprises an interface layer, located between the upper ELK dielectric layer 304 b and the first etch stop layer 302, which has a higher density and dielectric constant than the upper ELK dielectric layer 304 b. The lower density of the upper ELK dielectric layer 304 b causes the upper ELK dielectric layer 304 b to comprise a porous material having a low dielectric constant, k. In some embodiments, the upper ELK dielectric layer 304 b comprises a density in a range of between approximately 1.08 g/cm³ and approximately 1.15 g/cm³, which provides for a dielectric constant in a range of between approximately 2.4 and approximately 2.6.

A first metal layer 306 is disposed within the low-k dielectric layer 304. In various embodiments, the first metal layer 306 may comprise a metal interconnect layer 306 a and/or a metal via layer 306 b (i.e., a metal contact layer). Although FIG. 3 illustrates metal layer 306 as comprising both metal interconnect layers 306 a and metal via layers 306 b, it will be appreciated that a metal layer may comprise one or more the other, but not necessarily both. In some embodiments, the first metal layer 306 comprises a copper metal layer.

A first capping layer 308 is formed over the low-k dielectric layer 304 at a position between structures of the metal layer 306, resulting in structures of the first metal layer 306 being interspersed between the first capping layer 308 along a planar interface 310. In some embodiments, the first capping layer 308 may comprise a non-ELK film. In some embodiments, the first capping layer 308 may comprise a silicon nitride (SiN) film having a density of greater than 2.4 g/cm³. In other embodiments, the first capping layer 308 may comprise a silicon carbon nitride (SiCN) film having a density in a range of between approximately 1.5 g/cm³ to approximately 2.0 g/cm³. In yet other embodiments, a silicon dioxide (SiO₂) film having a density of approximately 1.5 g/cm³.

In other embodiments, the first capping layer 308 may comprise an ELK film having a density that is greater than a density of the underlying low-k dielectric layer 304 (e.g., the upper ELK dielectric layer 304 b). For example, in some embodiments, the first capping layer 308 may comprise a silicon oxycarbide (SiCO) film having a density in a range of between approximately 1.3 g/cm³ and approximately 1.4 g/cm³. Such densities provide for a dielectric constant having a value in a range of between approximately 2.8 and approximately 3.0. The higher density of the ELK film provides for a lower porosity than that of the underlying upper ELK dielectric layer 304 b.

A second capping layer 312 is disposed over the first metal layer 306. The first capping layer 308 has surface properties that improve selectivity of the second capping layer 312 between the first metal layer 306 and the first capping layer 308, so that the second capping layer 312 forms to a greater thickness over the first metal layer 306 than over the first capping layer 308. Improving the selectivity of the second capping layer 312, reduces diffusion from the second capping layer 312 to the underlying low-k dielectric layer 304, thereby improving TDDB and VDB reliability.

For example, in some embodiments, the second capping layer 312 may comprise a cobalt capping layer configured to improve electromigration at a grain boundary of the first metal layer 306. The first capping layer causes deposition of cobalt to proceed with a high selectivity that causes cobalt to easily accumulate over a metal surface but to not easily accumulate over the first capping layer.

In some embodiments, a second etch stop layer 314 may be disposed over the first capping layer 308 and the second capping layer 312. The second etch stop layer 314 has a first thickness t_(i) over the first capping layer 308 and a second thickness t₂ over the second capping layer 312. The first thickness t₁ is greater than the second thickness t₂ (i.e., t₁>t₂), since the second capping layer 312 has a greater thickness over the first metal layer 306. Additional metal layers 318 may be formed in additional dielectric layers 316 located over the second etch stop layer 314, in some embodiments.

FIG. 4 illustrates a graph 400 showing exemplary cobalt selectivity provided by surface properties of a disclosed first capping layer comprising an ELK film disposed over a blank semiconductor wafer. As provided herein, cobalt selectivity is the ratio of cobalt thickness over a copper metal layer to cobalt thickness over an extreme low-k (ELK) dielectric layer.

As shown in graph 400, the selectivity a first capping layer comprising an ELK film is directly proportional to the dielectric value of the ELK film. For example, an ELK film having a first dielectric value of k=2.6, corresponding to a first density, provides for a copper selectivity of 314. An ELK film having a second dielectric value of k=2.8, corresponding to a second density greater than the first density, provides for a copper selectivity of 4550.

It has been appreciated that the density of an ELK film has an inverse proportionality to the porosity of the ELK film and a direct proportionality to the dielectric value of the ELK film. Therefore, the selectivity a first capping layer comprising an ELK film is directly proportional to the density of the first capping layer, such that by using a first capping layer with a higher density (i.e., a higher dielectric constant) the selectivity of the first capping layer can be increased.

FIG. 5 is a flow diagram of some embodiments of a method 500 of integrated chip processing that improves back-end-of-the-line (BEOL) reliability.

While the disclosed methods (e.g., methods 200 and 500) are illustrated and described below as a series of acts or events, it will be appreciated that the illustrated ordering of such acts or events are not to be interpreted in a limiting sense. For example, some acts may occur in different orders and/or concurrently with other acts or events apart from those illustrated and/or described herein. In addition, not all illustrated acts may be required to implement one or more aspects or embodiments of the description herein. Further, one or more of the acts depicted herein may be carried out in one or more separate acts and/or phases.

At act 502, a first etch stop layer is formed over a semiconductor substrate.

At act 504, an extreme low-k (ELK) dielectric layer is formed over the first etch stop layer. In some embodiments, the ELK dielectric layer is formed by first forming a lower ELK dielectric layer having a first density, and subsequently forming an upper ELK dielectric layer over the lower ELK dielectric layer. The upper ELK dielectric layer has a second density that is less than the first density.

At act 506, a first capping layer is deposited over the ELK dielectric layer. The first capping layer 308 comprises surface properties that limit accumulation of a second capping layer (act 516) over the first capping layer. In some embodiments, the first capping layer may comprise an ELK dielectric film (e.g., SiCO) having a higher density (i.e., higher dielectric constant) than the underlying ELK dielectric layer. In other embodiments, the first capping layer may comprise a non-ELK dielectric material, such as silicon nitride (SiN), silicon carbide nitride (SiCN), or silicon dioxide (SiO₂).

In some embodiments, the first capping layer may be deposited by way of an “ex-situ process”, in which the first capping layer (e.g., SiN, SICN, or SiO₂) is deposited over a porous ELK film, after deposition of the ELK dielectric layer, by use of a different tool. In such embodiments, the substrate is exposed to an ambient environment between deposition of the ELK dielectric layer and the first capping layer. In other embodiments, the first capping layer may be deposited by way of an “in-situ process”, in which the first capping layer (e.g., the dense ELK film) is deposited over an underlying ELK dielectric layer by a same tool. In such embodiments, the first capping layer is formed without breaking the vacuum used to deposit the underlying ELK dielectric layer.

At act 508, a hardmask is selectively formed over the first capping layer. The hardmask is selectively formed to have a plurality of openings that expose the first capping layer at locations corresponding to one or more metal layer structures.

At act 510, the ELK dielectric layer and the first capping layer are selectively etched according to the hardmask to form a plurality of cavities within the substrate. The plurality of cavities extend though the first capping layer into the ELK dielectric layer, so that the plurality of cavities are disposed between openings in the first capping layer.

At act 512, a metal material is deposited over the substrate to fill the plurality of cavities.

At act 514, a chemical mechanical polishing (CMP) process is performed to remove excess metal material and the hardmask. The CMP process results in a planar surface comprising a plurality of metal layer structures disposed between the first capping layer.

At act 516, a second capping layer is selectively deposited over the substrate. In some embodiments, the second capping layer comprises a cobalt layer. The high selectivity of the first capping layer causes the second capping layer to form to a greater thickness over the metal layer structures than over the first capping layer. For example, in some embodiments, the second capping layer may be formed over the metal layer structures to a thickness that is greater than ten times the thickness of the second capping layer over the first capping layer.

At act 518, a second etch stop layer is optionally deposited over the first capping layer and the second capping layer, in some embodiments. The second etch stop layer has a substantially flat top surface and a first thickness over the first capping layer and a second thickness over the second capping layer.

It will be appreciated that method 500 may be iteratively performed to form a plurality of metal layers in a back-end-of-the-line (BEOL) stack.

FIGS. 6-13 illustrate cross-sectional views of some embodiments of an integrated chip (IC) layout whereon a disclosed method of integrated chip processing that improves back-end-of-the-line (BEOL) reliability is implemented.

FIG. 6 illustrates some embodiments of a cross-sectional view 600 corresponding to act 502. As shown in cross-sectional view 600, a first etch stop layer 302 is formed over a semiconductor substrate 102. In various embodiments, the semiconductor substrate 102 may comprise any type of semiconductor body (e.g., silicon, SiGe, SOI) such as a semiconductor wafer and/or one or more die on a wafer, as well as any other type of metal layer, device, semiconductor and/or epitaxial layers, etc., associated therewith. In various embodiments, the first etch stop layer 302 may comprise a layer of silicon nitride (SiN), silicon oxide (SiO), or silicon carbon nitride (SiCN).

FIG. 7 illustrates some embodiments of a cross-sectional view 700 corresponding to act 504. As shown in cross-sectional view 700, an extreme low-k (ELK) dielectric layer 304 is formed over the first etch stop layer 302. In some embodiments, the ELK dielectric layer 304 is formed by first depositing a lower ELK dielectric layer 304 a having a first dielectric constant over the first etch stop layer 302. An upper ELK dielectric layer 304 b, having a second dielectric constant less than the first dielectric constant, is subsequently deposited over the lower ELK layer 304 a. The lower dielectric constant of the upper ELK dielectric layer 304 b dielectric causes the upper ELK layer 304 b to have a lower density and a higher porosity than the lower ELK dielectric layer 304 a. In some embodiments, the lower and upper ELK dielectric layers, 304 a and 304 b, are deposited by way of a deposition process (e.g., a physical vapor deposition (PVD) process, a chemical vapor deposition (CVD) process, etc.) performed in a processing chamber held under vacuum.

FIG. 8 illustrates some embodiments of a cross-sectional view 800 corresponding to act 506. As shown in cross-sectional view 800, a first capping layer 308 is deposited over the ELK dielectric layer 304. The first capping layer 308 may be deposited by way of deposition process (e.g., a physical vapor deposition process such as sputtering) performed in a processing chamber held under vacuum.

In some embodiments, the first capping layer 308 may comprise a non-ELK film. For example, in some embodiments, the first capping layer 308 may comprise a silicon nitride (SiN) film having a density of greater than 2.4 g/cm³. In other embodiments, the first capping layer 308 may comprise a silicon carbide nitride (SiCN) film having a density in a range of between approximately 1.5 g/cm³ to approximately 2.0 g/cm³ In yet other embodiments, the first capping layer 308 may comprise a silicon dioxide (SiO₂) film having a density of approximately 1.5 g/cm³.

In other embodiments, the first capping layer 308 may comprise an ELK film having a density that is greater than a density of the underlying upper ELK dielectric layer 304. For example, in some embodiments, the first capping layer 308 may comprise an ELK film comprising a silicon oxycarbide (SiCO) film having a density in a range of between approximately 1.3 g/cm³ to approximately 1.4 g/cm³. In such embodiments, the density corresponds to a dielectric value in a range of between approximately 2.8 to approximately 3.0. The higher density of the ELK film, in relation to the ELK dielectric layer 304, provides the first capping layer 308 with a lower porosity than that of the ELK dielectric layer 304, thereby improving selectively and mitigating diffusion through the first capping layer 308.

FIG. 9 illustrates some embodiments of a cross-sectional view 900 corresponding to act 508. As shown in cross-sectional view 900, a hardmask 902 is selectively formed over the first capping layer 308. The hardmask 902 comprises a plurality of openings 904 that expose the underlying first capping layer 308 at positions corresponding to metal layer structures. In some embodiments, the hardmask 902 is selectively formed over the first capping layer 308 by forming a photoresist layer 906 over the hardmask 902. The photoresist layer 906 is patterned using a photolithography process, and then the hardmask 902 is exposed to an etchant 908, which removes the hardmask 902 at the plurality of openings 904. In some embodiments, the hardmask 902 may comprise titanium (Ti), aluminum (Al), tantalum (Ta), zirconium (Zr), hafnium (Hf), or some combination thereof, for example.

FIG. 10 illustrates some embodiments of a cross-sectional view 1000 corresponding to act 510. As shown in cross-sectional view 1000, an etchant 1002 is applied then to the substrate to selectively etch the first capping layer 308 and the ELK dielectric layer 304 according to the hardmask 902. The etchant 1002 selectively removes the first capping layer 308 and the ELK dielectric layer 304 at the one or more openings in the hardmask 902, resulting in a plurality of cavities 1004 that extend vertically through the first capping layer 308 into the ELK dielectric layer 304. In various embodiments, the etchant 1002 may comprise a wet etchant or a dry etchant.

FIG. 11 illustrates some embodiments of a cross-sectional view 1100 corresponding to act 512-514. As shown in cross-sectional views 1100, a metal material 1102 is deposited over the substrate. The metal material 1102 fills the plurality of cavities 1004 to form a plurality of structures in the metal layer 306. In some embodiments, the metal material 1102 may comprise copper deposited by way of deposition process (e.g., a physical vapor deposition such as sputtering). After the metal material 1102 has been deposited, a chemical mechanical polishing (CMP) process is performed to planarize the substrate along line 1104. The chemical mechanical polishing process removes the hardmask 902 and excess metal material 1102 without removing first capping layer 308 from over ELK dielectric layer 304, resulting in a planar interface having structures in the metal layer 306 horizontally interspersed between the first capping layer 308.

FIG. 12 illustrates some embodiments of a cross-sectional view 1200 corresponding to act 516. As shown in cross-sectional view 1200, a second capping layer 312 is formed over the substrate at a position over the metal layer 306. In some embodiments, the second capping layer comprises a cobalt capping layer that is separated from the ELK dielectric layer 304 by the first capping layer 308. The first capping layer 308 has a high selectivity, such that the thickness of the cobalt capping layer is much greater over the metal layer 306 than over the first capping layer 308. For example, in some embodiments, the thickness of the cobalt capping layer over the metal layer 306 is greater than ten times the thickness of the cobalt capping layer over the first capping layer 308.

FIG. 13 illustrates some embodiments of a cross-sectional view 1300 corresponding to act 518. As shown in cross-sectional view 1300, a second etch stop layer 314 is formed over the substrate at a position over the first capping layer 308 and the second capping layer 312. The second etch stop layer 314 is formed to have a first thickness t₁ over the first capping layer 308 and a second thickness t₂ over the second capping layer 312, wherein the first thickness t₁ is greater than the second thickness t₂ (i.e., t₁>t₂).

It will be appreciated that while reference is made throughout this document to exemplary structures in discussing aspects of methodologies described herein, those methodologies are not to be limited by the corresponding structures presented. Rather, the methodologies and structures are to be considered independent of one another and able to stand alone and be practiced without regard to any of the particular aspects depicted in the Figs. Additionally, layers described herein can be formed in any suitable manner, such as with spin on, sputtering, growth and/or deposition techniques, etc.

Also, equivalent alterations and/or modifications may occur to one of ordinary skill in the art based upon a reading and/or understanding of the specification and annexed drawings. The disclosure herein includes all such modifications and alterations and is generally not intended to be limited thereby. For example, although the figures provided herein are illustrated and described to have a particular doping type, it will be appreciated that alternative doping types may be utilized as will be appreciated by one of ordinary skill in the art.

In addition, while a particular feature or aspect may have been disclosed with respect to one of several implementations, such feature or aspect may be combined with one or more other features and/or aspects of other implementations as may be desired. Furthermore, to the extent that the terms “includes”, “having”, “has”, “with”, and/or variants thereof are used herein, such terms are intended to be inclusive in meaning—like “comprising.” Also, “exemplary” is merely meant to mean an example, rather than the best. It is also to be appreciated that features, layers and/or elements depicted herein are illustrated with particular dimensions and/or orientations relative to one another for purposes of simplicity and ease of understanding, and that the actual dimensions and/or orientations may differ from that illustrated herein.

Therefore, the present disclosure relates to a method and apparatus for improving back-end-of-the-line (BEOL) reliability.

In some embodiments, the present disclosure relates to an integrated chip. The integrated chip comprises a dielectric layer disposed over a semiconductor substrate, one or more metal layer structures disposed within the dielectric layer. A first capping layer is located over the dielectric layer at positions between the one or more metal layer structures. The first capping layer is located along an interface having the one or more metal layer structures interspersed between the first capping layer. A second capping layer is located over the one or more metal layer structures. An etch stop layer is arranged over the first capping layer and the second capping layer and laterally surrounding the second capping layer.

In other embodiments, the present disclosure relates to an integrated chip. The integrated chip comprises a first etch stop layer disposed over a semiconductor substrate. An extreme low-k (ELK) dielectric layer comprising a lower ELK dielectric layer is disposed over the first etch stop layer, and an upper ELK dielectric layer is disposed in contact with the lower ELK dielectric layer. One or more metal layer structures are disposed within the ELK dielectric layer. A first capping layer is disposed over the ELK dielectric layer at positions between the one or more metal layer structures and having a density greater a density of the upper ELK dielectric layer. A second capping layer is disposed over the one or more metal layer structures.

In other embodiments, the present disclosure relates to an integrated chip. The integrated chip comprises a dielectric layer disposed over a substrate, and one or more metal layer structures disposed within the dielectric layer. A first capping layer is located over the dielectric layer at positions between the one or more metal layer structures, and a second capping layer located over the one or more metal layer structures. The second capping layer has a greater thickness in areas overlying the one or more metal layer structures than in areas not overlying the one or more metal layer structures. 

What is claimed is:
 1. A method of forming an integrated chip, comprising: forming a first capping layer on a dielectric structure over a substrate; patterning the dielectric structure and the first capping layer to define cavities within the dielectric structure; forming a conductive material within the cavities; forming a second capping layer on the conductive material; and forming an etch stop layer along sidewalls and over an upper surface of the second capping layer, wherein the etch stop layer has a first thickness over the first capping layer and a second thickness over the second capping layer, wherein the first thickness is greater than the second thickness.
 2. The method of claim 1, further comprising: forming a patterned hard mask over the first capping layer, wherein the dielectric structure and the first capping layer are patterned with the patterned hard mask in place over the first capping layer.
 3. The method of claim 2, wherein the conductive material is formed over the patterned hard mask.
 4. The method of claim 3, further comprising: performing a chemical mechanical planarization process to remove a part of the conductive material and the patterned hard mask.
 5. The method of claim 1, wherein the first capping layer has a greater density than an uppermost surface of the dielectric structure.
 6. The method of claim 1, further comprising: forming a patterned hard mask over the first capping layer; patterning the dielectric structure and the first capping layer according to the patterned hard mask; and removing the patterned hard mask before forming the second capping layer and after patterning the dielectric structure and the first capping layer.
 7. The method of claim 1, wherein the first capping layer is formed in-situ with the dielectric structure.
 8. The method of claim 1, wherein the conductive material comprises copper and wherein the second capping layer has a greater thickness directly over the copper than outside of the copper, so as to cause the second capping layer to have sidewalls that are laterally aligned with sidewalls of the copper.
 9. The method of claim 8, wherein a first thickness of the second capping layer over the copper is greater than ten times a second thickness of the second capping layer outside of the copper.
 10. A method of forming an integrated chip, comprising: forming a dielectric structure over a substrate; forming a first capping layer over the dielectric structure and forming a hard mask on the first capping layer; patterning the dielectric structure and the first capping layer according to the hard mask to define cavities extending into the dielectric structure; forming a conductive material within the cavities and over the hard mask; removing the hard mask and a part of the conductive material to define a conductive interconnect, wherein the hard mask is removed after forming the conductive material; and forming a second capping layer over the conductive interconnect.
 11. The method of claim 10, further comprising: forming an etch stop layer over the first capping layer and over the second capping layer, wherein the etch stop layer has a first thickness over the first capping layer and a second thickness over the second capping layer, wherein the first thickness is greater than the second thickness.
 12. The method of claim 10, further comprising: performing a chemical mechanical planarization process to remove the part of the conductive material and the hard mask.
 13. The method of claim 10, wherein the first capping layer is formed in-situ with the dielectric structure.
 14. The method of claim 10, wherein the first capping layer has a first density and the dielectric structure has a second density that is less than the first density.
 15. A method of forming an integrated chip, comprising: forming a first capping layer on a dielectric layer over a substrate; performing an etch process on the dielectric layer and the first capping layer according to a hard mask, the etch process defining cavities that extend into the dielectric layer; forming a conductive interconnect within the cavities; removing the hard mask after performing the etch process; and forming a second capping layer on the conductive interconnect.
 16. The method of claim 15, further comprising: forming an etch stop layer over the first capping layer and over the second capping layer; and forming a second conductive interconnect that extends through a part of the etch stop layer to contact the second capping layer, wherein the second conductive interconnect has a bottommost surface that is below a top of the etch stop layer.
 17. The method of claim 15, further comprising: forming an etch stop layer over the first capping layer and over the second capping layer, wherein the etch stop layer has a first thickness over the first capping layer and a second thickness over the second capping layer, the first thickness is larger than the second thickness.
 18. The method of claim 15, wherein the conductive interconnect comprises copper and the second capping layer comprises cobalt.
 19. The method of claim 18, wherein the second capping layer has a greater thickness in areas overlying the copper than in areas not overlying the copper.
 20. The method of claim 15, further comprising: performing a mechanical planarization process to form a planar surface extending along top surfaces of the conductive material and the first capping layer. 